The goals of the PDT Physics core are to: 1) Provide Laser and other light source support for all Projects (1,2,3, and 4); 2) Perform high-quality and reliable in-vivo light dosimetry for Projects 1, 3, and 4; 3) Provide absolute light dosimetry standards for all projects (1,2,3, and 4). The physics core is responsible for calibrating absolute output power of laser sources so that it is traceable to National Institute of Science and Technology (NIST). The core will develop and improve in-vivo light dosimetry and various light detectors to achieve accurate light fluence measurement in PDT treatments, such as the Intraperitoneal PDT studies. Laser equipment operating at needed wavelengths (514, 532, 630, 652, 730 nm) for PDT are maintained, along with a light source continuously covering all wavelengths between 400 - 900 run for calibration of light detector and spectrometer and for measurement of tissue optical properties. The physics core ensures laser safety, maintains quality assurance of physics equipment, and works to characterize light fluence distributions in tissue. It helps to implement useful technologies developed in the physics project (Project 4) to routine clinical applications.